Im concered about my babys development?

my baby is 6 months old, the dr said she should be able to sit on her own by now, and i know that kids develope at different rates, but it king of looks like her spine is crooked, it is straight at the bottom and then going up towards the middle of her back there are two bumps that look like the balls of her spine ****** to the side, and when she tries to sit up she just plops down face first.